mk/module.mk: add Makefile to build kernel modules

Makefile example:

ifneq ($(KERNELRELEASE),)

obj-m := module01.o

else

top_srcdir	?= ../../../..
include $(top_srcdir)/include/mk/testcases.mk

REQ_VERSION_MAJOR	:= 2
REQ_VERSION_PATCH	:= 6
MAKE_TARGETS		:= test01 test02 module01.ko

include $(top_srcdir)/include/mk/module.mk
include $(top_srcdir)/include/mk/generic_leaf_target.mk

endif

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>
doc: Add documentation for module.mk

The example Makefile is taken from commit message from the previous
commit by Alexey Kodanev.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>